Where Is the Sacral Area on the Body?

The sacral area sits at the very base of your spine, just above the tailbone and between the two large hip bones. If you reach behind your back and press your fingers into the flat, slightly curved bony surface centered between your hip bones and below your belt line, you are touching the sacrum. It is a broad, triangular bone that forms the back wall of the pelvis, and it plays a surprisingly central role in everything from walking upright to bladder control. Despite being easy to find by touch, many people confuse it with the lower back or the tailbone, so understanding its exact boundaries and what it does clears up a lot of common questions about pain, posture, and medical procedures in that region.

What the Sacrum Actually Is

The sacrum is not a single vertebra but a set of five vertebrae (labeled S1 through S5) that gradually fuse together as you grow. At birth, these are still separate bones connected by cartilage. Fusion happens over many years, and the last segments to fully knit together are S1 and S2 at the top of the sacrum, which may not achieve complete bony fusion until around the late twenties in men and mid-twenties in women.¹ The finished product is a solid, wedge-shaped bone roughly the width of your hand, wider at the top and tapering to a point at the bottom where it meets the coccyx (tailbone).

An important detail for orientation: the sacrum is not vertical. It angles backward, creating a curve that helps position the spine’s weight over the legs. The front (anterior) surface of the sacrum faces the inside of your pelvis and is smooth and concave. The back (posterior) surface, the one you can feel through the skin, is bumpy with ridges where the fused vertebrae left behind bony remnants. At the very bottom of the back surface is a small opening called the sacral hiatus, which is the gap where the bony roof of the sacral canal does not fully close. This opening matters in medicine, as we will see later.

How to Find It on Yourself

The simplest way to locate your sacrum is to find the two small dimples on your lower back, sometimes called the “dimples of Venus.” These skin indentations sit roughly over the points where the sacrum connects to the hip bones. However, using those dimples as a precise spinal landmark is unreliable. A study of scoliosis patients found that the widely taught rule of sacral dimples sitting at the S2 vertebral level held true in only about 7% of subjects.² So while the dimples put you in the right neighborhood, the exact vertebral level underneath varies quite a bit from person to person.

A more reliable approach: place your hands on your hips with your thumbs pointing backward. Your thumbs will naturally rest near the top of the sacrum. The bone extends downward from there for about four to five inches until it ends at the small, knobby coccyx. The sacrum is wider than most people expect, roughly as wide as the space between the two bony bumps you can feel at the back of your pelvis (the posterior superior iliac spines, which are where those skin dimples form).

The Sacroiliac Joints and Why They Matter

On each side, the sacrum connects to the large ilium bone of the pelvis through the sacroiliac joint. These joints are not flashy or dramatically mobile like a knee or shoulder. They allow only a few degrees of movement. But they serve as the critical bridge between your spine and your legs, transferring the entire weight of your upper body down into the pelvis and then into the ground through your legs.³ They also handle large bending and compression forces during activities like walking, running, and lifting.⁴

Because the sacroiliac joints carry so much load, they are reinforced by some of the strongest ligaments in the body. The joint surfaces themselves are irregular, with interlocking ridges and grooves that help prevent slipping. In women, these joints tend to be slightly more mobile and have flatter surfaces, which accommodates the widening of the pelvis during childbirth. This difference also helps explain why sacroiliac joint problems are more common in women.

What the Sacral Nerves Control

The spinal cord itself usually ends higher up, around the first or second lumbar vertebra. But a bundle of nerve roots continues downward through the sacral canal like a horse’s tail (which is why it is called the cauda equina). These sacral nerve roots exit through small holes called foramina on the front and back of the sacrum and fan out to control a surprising range of functions.

The upper sacral nerves (S1, S2) contribute to leg movement and sensation. When an S1 nerve root is compressed, the pain pattern is inconsistent with the neat skin-zone maps you see in textbooks. One study found that patients with confirmed S1 nerve root compression placed most of their pain markings on the back of the leg, with only about 4% reporting any pain on the front, and no patient placed the majority of their pain within the textbook S1 skin zone.⁵ The takeaway: sacral nerve pain does not follow the clean dermatomal diagrams you might see online.

The lower sacral nerves (S2, S3, S4) control the bladder, the rectum, and sexual function. Stimulating the S3 nerve root, for example, can produce strong contractions of the left colon, strong enough to move its contents toward the rectum.⁶ This is why sacral nerve stimulators are now used clinically for people with bladder or bowel control problems. It is also why injuries to the lower sacrum can cause dysfunction in these areas even when leg strength remains intact.

Sacral Dimples in Newborns

Parents sometimes notice a small pit or dimple at the base of their newborn’s spine and worry about a spinal cord problem. Simple sacral dimples, meaning shallow, midline, and close to the anus, are common and almost always harmless. One study of 151 infants who had spinal ultrasounds because of a sacral dimple found that 80% had completely normal results. Among those with abnormalities on ultrasound, only about 1% ultimately needed surgery.⁷ The researchers noted that routine ultrasound screening for simple sacral dimples, when no other warning signs are present, tends to generate unnecessary costs and anxiety without catching meaningful problems.

A different picture emerges when a sacral dimple is accompanied by other skin markers like a tuft of hair, a skin tag, a deep sinus tract, or skin discoloration. In those cases, the risk of an underlying spinal cord abnormality is higher, and imaging is warranted. A study comparing different skin markers found that a dermal sinus (a deeper, tunnel-like opening) carried a much higher risk of spinal cord problems than a simple dimple alone.⁸ The distinction matters: a shallow dimple by itself is common and benign, but additional markers change the picture.

Sacroiliac Joint Pain and Why It Gets Misdiagnosed

Sacroiliac joint dysfunction is one of the more under-recognized causes of low back pain.⁹ It typically causes pain on one side of the lower back or buttock, sometimes radiating into the groin or down the back of the thigh. Because this overlaps heavily with disc herniation, piriformis syndrome, and hip joint problems, it frequently gets blamed on something else. Making diagnosis harder, physical examination tests for the sacroiliac joint have inconsistent reliability. A systematic review found that the agreement between different examiners assessing sacroiliac joint motion varied widely, and one study concluded that sacral position could not be reliably assessed during trunk movement.¹⁰

The gold standard for confirming sacroiliac joint pain is a diagnostic injection: if numbing the joint relieves the pain, the joint was the source. Treatments range from physical therapy focused on stabilizing the pelvis, to joint injections, to surgical fusion in severe cases. The key practical point is that if you have persistent one-sided pain in the sacral area that has not responded to typical lower-back treatments, sacroiliac dysfunction is worth investigating specifically.

Sacral Insufficiency Fractures

The sacrum can fracture without any memorable injury or fall, particularly in older adults with osteoporosis. These are called insufficiency fractures, and they happen when normal daily forces exceed the weakened bone’s ability to hold up.¹¹ They cause deep, aching pain in the sacral area or low back that worsens with standing and walking. Because the pain mimics many other conditions, sacral insufficiency fractures are easily missed. Standard X-rays are poor at detecting them; one study found that plain films showed evidence of the fracture in only 4 out of 20 confirmed cases. Bone scans caught all of them, and CT was moderately helpful.¹²

The typical patient is a postmenopausal woman with osteoporosis, though radiation therapy to the pelvis (for cervical or rectal cancer, for example) also weakens the bone and raises the risk. Most sacral insufficiency fractures heal with rest, pain management, and treatment of the underlying bone loss, but they can take weeks to months to resolve fully.

Pressure Ulcers on the Sacrum

If you or someone you know has been hospitalized for an extended stay, you may have heard nurses talk about “sacral skin breakdown.” The sacrum is one of the most common sites for pressure ulcers because when you lie on your back, the skin and tissue over the sacrum get compressed between the bone and the mattress.¹³ The sacrum has relatively little padding over it compared to, say, the buttock muscles on either side, so prolonged pressure cuts off blood flow and damages tissue.

This is why hospitals use specialized mattresses, repositioning schedules, and skin assessments focused heavily on the sacral area. For people caring for bedridden family members at home, checking the skin over the sacrum every few hours and ensuring the person changes position regularly are the most effective prevention measures.

Medical Procedures That Target the Sacrum

The sacral hiatus at the bottom of the sacrum provides a natural entry point for caudal epidural injections. A needle is inserted through this small opening to deliver anesthetic or steroid medication into the sacral canal, bathing the nerve roots in the medication.¹⁴ This technique is commonly used in children undergoing surgeries below the waist and in adults for chronic lower-back or leg pain. However, the anatomy of the sacral hiatus varies considerably between individuals. The hiatus can differ in shape, size, and depth, and if it is too shallow (less than about 3 mm deep), inserting the needle may fail entirely.¹⁵ Ultrasound guidance has become more common to compensate for this variability.

A question that comes up around the sacral area involves lower-back tattoos and epidural anesthesia during labor. The concern is that inserting a needle through tattooed skin could push pigment particles into the spinal space. While the risk remains debated, some clinicians consider it prudent to insert the needle through an uninked spot or use an alternative approach when possible.¹⁶ In practice, epidural needles for labor go into the lumbar spine rather than the sacrum itself, so a tattoo would need to extend up to the lumbar region to be relevant. Purely sacral tattoos are not usually in the path of a standard labor epidural.

Tumors of the Sacrum

Sacral tumors are uncommon but worth knowing about because they can mimic more benign conditions for months before being diagnosed. The most distinctive sacral tumor is a chordoma, a slow-growing cancer that arises from remnants of the notochord, the embryonic structure that originally guided spinal development. Chordomas have a particular preference for the sacrum and tend to involve the S2 and S3 levels.¹⁷ They grow slowly but are locally aggressive and difficult to cure. Surgery with wide margins is the primary treatment, but achieving adequate margins often means sacrificing sacral nerve roots, which can cause loss of bladder, bowel, or sexual function. Chordomas also respond poorly to conventional radiation and chemotherapy, and recurrence rates remain high even after aggressive surgery.¹⁸

Other tumors that can appear in the sacrum include giant cell tumors, schwannomas, and metastatic deposits from cancers elsewhere in the body. Any persistent, unexplained pain deep in the sacral area, especially if it worsens at night or does not improve with rest, warrants imaging to rule out a mass.

Sacral Agenesis

In rare cases, part or all of the sacrum fails to develop before birth, a condition called sacral agenesis. This can range from a missing coccyx and lower sacral segments (which may cause few symptoms) to complete absence of the sacrum, which causes severe problems with leg function, bladder and bowel control, and spinal stability. The only well-established risk factor for sacral agenesis is maternal diabetes, which significantly raises the incidence.¹⁹ Milder forms sometimes go undetected until later in childhood or even adulthood, when unexplained bladder or bowel issues prompt imaging.

Variations in Sacral Anatomy

Not every sacrum is made of five vertebrae. One of the more common anatomical variations is the incorporation of an extra vertebra from above (lumbarization, where the first sacral vertebra behaves more like a lumbar vertebra) or from below (sacralization, where the last lumbar vertebra fuses into the sacrum). A morphological study found that when the sacrum has six elements instead of five, it more often incorporates the coccyx rather than the fifth lumbar vertebra, and coccygeal fusion was present in about a third of those cases.²⁰ These variations are usually discovered incidentally on imaging and rarely cause symptoms, but they can complicate surgical planning or injection procedures when the expected anatomy does not match what is actually there.

Why the Human Sacrum Looks the Way It Does

Compared to our closest primate relatives, the human sacrum is distinctly wider and incorporates more vertebrae. In great apes, the sacrum is narrower and its wing-like extensions (alae) are small. In humans, those wings are large and flared, providing a broad articulation surface with the pelvis. A comparative study of primate sacra concluded that the human tendency to incorporate extra lumbar vertebrae and to develop wide sacral wings provides the enhanced pelvic connection needed to support upright trunk weight and stabilize the body during bipedal walking.²¹ In essence, the sacrum widened and strengthened as our ancestors began walking on two legs, because the forces passing through it during upright movement demanded a more robust skeletal anchor.

Sitting, Posture, and the Sacral Angle

When you sit down, your pelvis rotates backward and your sacrum tilts. This shift flattens the natural curve of the lower spine and puts the lumbar discs under more stress than standing does. A radiographic study measuring spinal angles across different office chair configurations found that sitting involved roughly 70% of the spine’s maximum bending range regardless of the chair features tested. A tilted seat pan (the part you sit on angled slightly forward) improved pelvic posture more than lumbar support alone, though no single chair feature eliminated the fundamental problem: sitting pushes your spine close to its end range of flexion.²² If you spend hours in a chair daily, this explains why occasional standing or walking breaks help more than any ergonomic chair feature on its own. The sacral angle changes during sitting are not something a cushion can fully reverse; reducing total sitting time matters more.